Coated paper and method of making the same



Patented Sept. 10, 1940 UNITED STATES- COATED PAPER AND METHOD OF MAK- ING THE SAME William J. Montgomery and Donald B. Bradner,

Hamilton, Ohio,

Paper and Fibre Company,

corporation of Ohio assignors to The Champion Hamilton, Ohio, a

No Drawing. Application January 26, 1938, Serial No, 187,077

2 Claims.

This invention relates to a coated paper and particularly to paper having a cast coating applied by the processes described in the patent to Donald B. Bradner, issued July 2, 1929, No.

5 1,719,166. The above designated patent discloses novel methods of applying a coating composition to paper stock, the essential step in the processes being the solidification of a plastic or liquid coating composition in contact with a 0 smooth non-adhering surface, such as the periphery of a drum or surface of a belt, whereby the finished surface of the paper constitutes a cast reproduction of the surface against which the composition was solidified.

Prior to the invention of the above-mentioned Bradner patent the universal practice of coating paper had been to apply the coating composition, allow it to dry, and then subject the paper to a calendering or similar mechanical operation to impart a high degree of polish and smoothness to the paper. In order to secure an acceptable finish for the coated paper it has been customary heretofore to apply a coating composition in quantity amounting to an average of pounds per ream of 500 sheets x38 inches, which is equivalent to 4.5 pounds per thousand square feet. While the weight of coating composition necessary to give the most satisfactory finish to the paper varies with the nature of the coating composition, the type paper stock employed, and other conditions, the above figure is typical of the general practice, and in the practice of the processes described in the abovementioned patent, it was contemplated that the coating compositions would be used in the same amounts as employed in carrying out the old processes.

We have discovered that when a coating composition is cast upon certain types of paper stock by the process described in the abovementioned patent, a greatly superior finish may be had with a very much smaller amount of coating composition per unit of area than heretofore used.

The cast coated paper produced by the Bradner process with coating compositions in the amounts heretofore used have frequently suffered from microscopic defects which detracted 5 from the appearance and interfered with fine printing. Under the microscope these defects appear in the form of minute holes and cracks and as shallow trenches that follow certain of the surface fibers of theunderlying paper.

II Also, the coating may be of irregular thickness.

with thick and thin areas spaced a millimeter or two apart.

When viewed with the naked eye these microscopic defects detract from the finish 'of the paper and, also, by reason of their irregular dis- I tribution, give it a mottled or streaked appearance. 1

The paper may also have the cockled appearance.of air dried bond paper as contrasted with a fiat calendered paper.

Using a smooth paper stock, the quality of the product, due to these defects, becomes progressively poorer as the weight of coating applied by the Bradner method is diminished, until a point is reached, which varies with different coating compositions and different paper stocks, where the'quallty begins to improve; and at very light weights of coating most of these defects become markedly reduced or are entirely elimi nated. By very light weights of coating we mean coatings on the dry basis of the order of about a pound per 1000 square feet per side,-and not exceeding 2 pounds per 1000 square feet.

With super-calendered coated paper, on the contrary, the quality of the papercontinues to become progressively poorer as the weight of the coating is reduced, and such paper with very thin coatings such as here contemplated, is not comparable in quality with either the supercalendered coated paper with coatings of usual thickness or the paper with the thin cast coatings.

For coatings of light weight, the characteristics of the paper stock must be taken into consideration for the reason that light weight coatings of this character will not completely cover a rough stock and furthermore are not opaque even when the mineral content is comparatively high and consequently if the paper stock to which the coating is applied presents an uneven or unsuitable color or texture, the finished product will not be acceptable even though in smoothness, gloss and absorptivity of the surface it'is of acceptable standard. There are a number of types of smooth-finished paper which may be used with satisfactory results with coatings of light weight.

The following is a specific example which has been found satisfactory.

A well formed paper coating stock was heavily super-calendered between paper and metal rolls. One and one-half pounds per 1000 square feet of a mixture containing casein and 45% precipitated calcium carbonate with 1% ammonium stearate was applied and given a cast 86 surface in the manner described. This paper had a uniform, high gloss, coating which was practically free from surface imperfections. By virtue of its very low absorbency to printing ink, brilliantglossdnk effects were obtained upon it.

A smooth, closed up, well formed stock, made in accordance with U. S. Patent No. 2,088,093

issued August 3, 1937 to L. C. Gates, was

coated with 1.1 pounds (dry ,basis) per 1000 square feet of aqueous mixture of parts of coating clay, 20 parts of casein, and 1 part ammonium stearate, and dried in contact with a heated mirror-finished chromium plated surface. The resulting paper had a verysmooth surface of quite high gloss, absorptive to printing ink, and. having practically no surface imperfections that would interfere with high grade printing.

We have found paper finished according to the special calendering processes disclosed in said Cates patent particularly suitable for cast coatings of light weight. With understock of this character and a coating composition having a substantial excess of adhesive over that necessary to secure the adhesion of the coating composition to the paper a satisfactory product is obtained with as little as .8 pound of coating composition per thousand square feet of surface.

Smooth finished paper, provided it is of good quality and uniform color, may generally be employed with satisfactory results. I

The invention of this application is useful for producing coated papers of deep color. The dye or colored pigment may be incorporated in the coating, or the paper stock may be colored and a thin, transparent, cast coating applied, or the color may be incorporated in both the paper stock and the coating. The following described procedure has been used with highly satisfactory results.

The paper used was made from a mixture of sulfite and soda pulp, sized with rosin size, and dyed a dark brown. It was super-calendered according to the method described in the abovementioned Cates patent, whereby the paper obtained an unusually smooth, dense, well closed up surface. This stock was coated with about 1.5 pounds (dry basis) per thousand square feet of a coating containing 78% of shellac, 4% of sulphonated castor oil, and 18% of dye, in aqueous ammonia dispersion, and dried in contact with a heated mirror-surfaced chromium plated drum. The product had an exceptionally high and beautiful gloss and was well suited for gloss ink printmg.

Generally speaking, the quantity of coating composition per unit of area required to give a satisfactorycoating is greater with a larger percentage of filler, butnot in direct proportion. That is to say, a coating of casein or other adhesive without any filler may be used in less quantity than a coating composition containing an admixture of filler. However, the additional amount of coating required with a higher percentage of filler is less than the amount of filler added and consequently comparable effects may be had with a lesser total amount of adhesive by the incorporation of suitable filler than when adhesive without filler is employed.

For many purposes it is desirable that the quantity of adhesive be in excess of that required to prevent "picking of the coating from the paper by the action of the printing press. As described in our co-pending application filed concurrently herewith, coatings which contain no excess adhesive are quite absorptive to printing ink and with thin coatings such as covered by the present application, a high degree of ink absorption detracts from gloss ink printing to even a greater extent than with thicker coatings. This is probably due to the fact that the percentage variation in thickness of the thin coating (on the same paper stock) is much greater than with the thicker coating and hence the penetration of the ink will vary to a greater I extent than with the thicker coatings.

Thin cast coatings such as herein described may be applied by the processes described in the Montgomery and Bradner Patent No. 2,029,273, issued January 28, 1936, with equally good results.

While our invention is particularly applicable to coatings cast from aqueous solution, it is not limited to the use of such solutions, nor is the invention in any way limited to the specific fillers, pigments, and other ingredients of the coating compositions mentioned in the above examples, or to the particular adhesives therein mentioned.

Other adhesive substances previously known in the art or disclosed in the above mentioned patents may be employed.

The expression cast surfaced layer is used in the appended claims to designate a coating which has been set from a plastic condition in contact with a solid, nonadhering casting surface described in the patents, to Bradner, No. 1,719,166, or, to Bradner and Montgomery, No. 2,029,273, so that the surface of the coating has substantially the same finish as the casting surface itself.

We claim:

1. In the making of coated paper the process which comprises applying directly to the surface of smooth finish uncoated paper stock a layer of liquid paper coating composition comprising an aqueous suspension of mineral filler and adhesive in an amount in the order of one pound, and not exceeding two pounds, per thousand square feet of surface, dry basis, and drying said layer of coating composition with its surface in contact with a heated finishing surface until the major portion of the liquid has been evaporated through theuncoated side of the paper stock and the coating material is no longer adherent to the finishing surface.

2. Coated paper made by the process of claim 1 wherein the coating material is in the form of a continuous layer not exceeding two pounds, dry basis, per thousand square feet of surface and has a cast surface finish substantially free from minute and microscopic surface defects.

WILLIAM J. MONTGOMERY- DONALD B. BRADNER. 

